Gradients in brain organization

Complementary to the progress that has been made in the dentification and mapping of discrete brain areas ( Amunts et al., 020 ; Genon et al., 2021 ) and macroscopic functional communities Eickhoff et al., 2018b ; Schaefer et al., 2018 ; Yeo et al., 2011 ), recent ears have seen a rise of new methods and applications to study spaial transitions — or gradients — of brain organization ( Haak and Beckann, 2020 ; Haak et al., 2018 ; Huntenburg et al., 2018 ; Margulies et al., 016 ; Vos de Wael et al., 2020 ). Already emphasized by early analyes of post mortem cytoarchitecture in humans and non-human primates Bailey and Bonin, 1951 ; Sanides, 1962 ; Von Bonin and Bailey, 1947 ), ecent neuroimaging and network neuroscience studies have made sigificant progress in the mapping of spatial gradients in the human and on-human brain. Such gradients have been described across multiple aspects of brain rganization, including MRI-based measures of cortical morphology nd microstructure ( Demirtas et al., 2019 ; Huntenburg et al., 2017 ; aquola et al., 2019b ; Wagstyl et al., 2015 ), indices of functional and tructural connectivity ( Margulies et al., 2016 ; Oligschläger et al., 2017 ; os de Wael et al., 2021 ), as well as task-based activation and deactivaion patterns ( Caciagli et al., 2021 ). Overall, these studies suggest conuent spatial trends of neural organization in which structure-function elationships can be investigated. For example, one key study in humans nd non-human primates based on resting-state fMRI analysis demontrated a principal axis of functional differentiation between sensory nd heteromodal systems ( Margulies et al., 2016 ), which closely coresponded to models of the primate cortical hierarchy ( Mesulam, 1998 ) nd furthermore showed that heteromodal networks such as the deault mode network are maximally spatially distant from sensory sysems. In parallel, a mounting literature has shown evidence for simiar gradients at the level of gene expression patterns ( Burt et al., 2018 ; oulas et al., 2019c ; Seidlitz et al., 2019 ; Vogel et al., 2020 ) as well s receptor architecture ( Hansen et al., 2021 ), suggesting that gradints may offer a framework to interrogate the interplay between brain tructure, metabolism, and function. Studying microstructural and funcional gradients in developmental and aging cohorts has furthermore rovided insights into lifespan changes in the spatial layout of brain rganization ( Baum et al., 2020 ; Bethlehem et al., 2020 ; Paquola et al., 019a ; Park et al., 2021a ; Sydnor et al., 2021 ). Such approaches may ulimately lead to better understanding of brain changes seen in prevalent eurological and neuropsychiatric conditions ( Benkarim et al., 2021 ; adaie et al., 2021 ; Li et al., 2021 ; Park et al., 2021b , 2021d ; Tian et al., 019 ; Dong et al., 2020 ). Finally, by providing a continuous coordiate system to characterize topographic organization and structure-